Production of low-carbon steel for structural and similar purposes



July 13, E926. 11,592,1 1

' W. J. CROOK PRODUCTION OF LOW CARBON STEEL FOR STRUCTURAL AND SIMILAR PURPOSES Filed Jan. 17, 1925 Patented July 13, 1926.

UNITED STATES PATENT OFFlCE.-

WELTON J. CROOK, 0F STANFORD UNIVERSITY, CALIFORNIA, ASSIGNOR TO IPACIFIC COAST STEEL COMPANY, OF SAN FRANCISCO, CALIFORNIA, A CORPORATION OF camronurn.

PRODUCTION OF LOW-CARBON STEEL FOR STRUCTURAL AND SIMILAR PURPOSES.

Application filed January 17, 1925. Serial No. 3,187.

This invention relates to the production of low carbon steel for structural and similar purposes.

Steel used for structural and similar pur-v poses requires high limits of elasticity and ductility. a satisfactory degree, must contain a relatively small amount of carbon, usually not exceeding 0.25 per cent, and not more than 1 per cent manganese. While an increase in the carbon content will raise the yielding point and maximum stress of the steel, it will reduce the elongation and reduction area to such an extent as to cause brittleness; likewise, an excess of manganese in steel will raise the strength, but in percentages above 1 per cent it begins to produce undue hardness and brittleness. Hence the carbon content of steel used for structural purposes is seldom greater than 0.25 per cent and the manganese content does not exceed 1 per cent and frequently falls below that amount. It is to this character of steel that the present invention relates.

In practice, the steel used for the above purposes is hot-rolled, forged or pressed, and in this way there is brought about a certain grain refinement which greatly increases the strength of the steel, but the strength which it has been possible to obtain in this manner does not in commercial practice exceed 45,000 pounds per square inch elastic limit.- Heattreatment of such low carbon steel for the purpose of increasing the strength is never resorted to in practice, since it is generally believed that the useful properties can not be increased through suchv treatment.

I have discovered that by proper heat treatment of low carbon steel such as is used for structural and similar purposes, including castings within the abovelimits of carbon and manganese used in construction of machinery, industrial equipment, railroadequipment, automotive equipment, and the numerous destinies of low carbon castings, the elastic limitand tensile properties can be enormously increased without affecting the other properties, such as ductility, necessary in steel of this character. As above "stated, the best grades of steel for construc tion purposes heretofore possible have had an elastic limit. of 45,000 pounds per square- Steel, to possess these qualities in inch, whereas I have produced low carbon steel suitable for structural and similar purposes and having an elastic; limit of 60,000 pounds per square inch and over.

Generally stated, the method or process which I make use of consists merely of heating normal steel containing not more than 0.25% carbon and between 5% and 1% manganese, to a temperature above the transformation point-or so-called critical range, quenching the same in water, oil, or other suitable quenching medium, reheating or drawing back the steel to a temperature below transformation point, and either allowing the steel to cool gradually on again quenching.

In practicing this process I have made numerous tests, the results of which have been carefully recorded. For example, one series of six tests was made on bars cut from two-inch by one-half inch flat stock having the following analysis:

Per cent. Carbon .14 Manganese .58 Phosphorus .022 Sulphur .045

Before being subjected to heat treatment the steel was tested and showed an elastic limit of 40,000 pounds per square inch, elongation in eight inches-of 32%, reduction area 60%, and Brinell hardness 137. All of these bars were first heated to a temperature of 1600 Fahrenheit and held at this temperature for approximately thirty minutes, and thereafter quenched in water. The Brinell hardness after being quenched varied from 196 to 228. Two of the bars were reheated to temperatures of 1000 Fahrenheit and held at such temperature for twenty minutes. After cooling in the atmosphere theywere tested and shown to have an elastic limit of 65,000 pounds to the square inch, as against 40,000 pounds to the square inch possessed by these same bars before being subjected to this treatment. Their ductility was somewhat less than that of the original steel, since the bars would not bend flat, although they could be bent around a one-half inch pin without crackutes. When tested they showed elastic limits of approximately 57,500 pounds to the square inch, and a sufficient increase in ductility to permit of their being bent flat. The percentageelongation and percentage of reduction area, which are the measure of ductility, in the case of these last-named bars showed an average of 22% elongation in relation to an initial distance of eight inches and an average reduction area of about 64%.

The following is a table setting forth the details of the tests above mentioned:

Orig. bar No. 1 2 3 4 5 6 Temp.ofquench 1600 F. 1600 F. 1600 F. 1600 F. 1600 F. 1600 F. Time of quench. 30 M. 30 M. 30 M. 30 M. 30 M. 30 M. Brinell hardness. 228 228 196 228 217 202 Temp. of draw-. 1000 F. 1000 F. 1200 F. 1200 F. 1200 F. 1200 F. Time of draw..- 20 M. 20 M. 20 M. 20 M. 20 M. 20 M.

Brinell alter I draw 212 187 179 207 179 179 Ult. strength 1 sq. 62260 82680 83700 76450 76470 77000 76580 E astlc limit r sq. in 40200 64310 65500 57480 57040 57330 57460 E ongation in 8 inches 32 20. 5 20. 5 24 21 23 22 Elongation in 2 inches 48 39 36- 40 Reduction area- 60 57 64 65 63 64 Bend Flat 5m. kin. Flat. Flat. Flat. Flat.

pin pin.

The temperatures and times mentioned above are not to be considered as a limitation, but merely as illustrative of what appeared to give the best results for the particular purpose at hand. Therefore, in practicing my process it will be borne in mind that some temperature above the critical range may be selected for the first heating. In the low carbon steel with which the present invention is concerned, the critical ranges will occur between 1300 and 1700 Fahrenheit. Also, the time of heating will determine to some extent the exact temperature, since by increasing the time of the heating a lower temperature will suffice. All of this is well understood by those skilled in the art, and the various times and temperatures will be modified according to the experience of the individual heat-treatment man and the methods of heat application.

Photomicrographs of sections of two specimens of steel, enlarged 1,000 times, are reproduced in the accompanying drawing, wherein I Fig. 1 shows the original steel before heat treatment;

Fig. 2 shows the steel after being heated for fifteen minutes at 1600 Fahrenheit,

uenched, and drawn at 1200 Fahrenheit or twenty minutes.

A consideration of these reproductions from photomicrographs will serve to demonstrate the change brought about in the physical properties of the material by the present method of treatment.

It was found that the structure after being quenched from 1600 Fahrenheit had an appearance somewhat like that of a slowly cooled pearlitic steel, but it was not pearlitic, since pearlitecontains 85% carbon, and therefore in a section of .12% carbon steel the area occupied by pearlite would be 14%. whereas a photomicrograph showed this new structure covering nearly 100% of the area.

It is considered that heating to 1600 Fahrenheit for fifteen or twenty minutes has converted the iron to the gamma state and enabled the cementite, of the original 14% pearlite, to break down and form a solid solution of carbon in gamma iron known as austenite. In order that a homogeneous austenite may form, the carbon from the pearlite must have time to migrate or diffuse at least across the distance equal to onehalf the average diameter of one ferrite grain as measured in the original steel. In

the case under discussion the average grain.

diameter of the ferrite grains in the original untreated steel was about 0.0200 mm, onehalf of which distance is 0.01 m.m., or

In the cementation process, where carbon is caused to diffuse into the steel under the influence of heat, it is found that the carbon migrates into the steel at the rate of about in 24 hours, or 000026 per minute. Experiment has shown that, in the case of the diffusion of the carbon from pearlite in this low carbon steel, it takes at least 15 minutes for the diffusion to take place over about 0.0008. This may be explained by the fact that, as the carbon from the pearlite areas diffuses, it becomes more and more dilute in the gamma iron so that the diffusion pressure becomes less and less. In the case of higher carbon steels, the carbon not only has less distance to travel (in the case of pearlitic steel, almost no distance at all), but the diffusion pressure decreases less rapidly. It is therefore apparent, from the foregoing, that the time for which the low carbon steel is held at the quench temperature is extremely important if the best results are to be realized.

The austenite formed when the quench temperature has been sufficiently high and has been maintained for long enough time, e. g., 1600 Fahrenheit for twenty minutes, so that the carbon is completely diffused, cannot be preserved as such, with low carbon steel, unless the cooling is very rapid, and in the presentprocess the austenite during quenching transforms over into martensite.

This martensitic structure has very different physical properties from that found site, in some specimens, was found to be over 400 Brinell, and its ultimate strength in some cases exceeded 100,000 pounds per square inch. In this form the steel would not be useful for structural and other hereinbefore mentioned purposes, and its physical properties must be modified somewhat by subsequent heat treatment.

A gradual structural change takes place when the quenched steel 'is reheated to various temperatures. The dark martensitic areas are gradually replaced by what is apparently carbonless iron. The martensite seems to be gradually pushed to What are apparently the forerunners of grain boundaries, which become distinct when a reheat.-

ing temperature of 1000 Fahrenheit is reached. There does not seem to be any development of the transition products, such as sorbite and troostite, which are met with in the heat treatment of high carbon steels.

When the reheating is carried over the critical point (1340 Fahrenheit), say to 1400 Fahrenheit, followed by slow cooling, there is a formation of ferrite polyhedra and the concentration of carbon,on the grain boundaries. into pearlite areas. The martensite on the grain boundaries bodily transforms into pearlite. The steel will then be back where it started before any heat treatment.

ltshould be borne in mind that the present inventlon has to do, not with an alloy steel, but with a plain carbon steel of low carbon content. I am aware that alloy steels have been heat-treated for urposes of increasing their useful properties, also that plain-carbon steels, generally referred to as tool steels and containing considerably higher carbon content than steels used for structural purposes, have been heattreated for hardening'and tempering. However, so far as I am aware, plain carbon steel possessing the high elastic limit and ductility mentioned herein has not heretofore been produced, nor have carbon steels containing less than 0.25% carbon been subjected to heat-treatment for the purpose of increasing those qualities desired in a structural steel. By subjecting such steel to the present process the tensile strength thereof can be increased approximately 30%, and thus an enormous saving can be brought about through the smaller quantity of material required; also the reduction in weight of the steel will be an important factor in many situations.

In order to insure successful results in the use of my process it is essential that migration of the carbon across the ferrite grain should take place during the first heating. This I have found will not occur unless the proper amount of manganese is present in the original steel.

.40% manganese is present an increase in t the quenching temperature causes a decrease in the ultimate strength. The tendency for a change in rate of this decrease is delayed until a temperature of about 1600 F. is reached. When 63% manganese is present an increase in quench temperature results in a rapid increase in ultimate strength. In other words, heating to temperatures of 1400 F. or more from any period of time ranging from ten to thirty minutes, will bring about an increase in the strength of low carbon'steel containing more than .5% of manganese; 'whereas similar treatment of low carbon steel containing less than .5% manganese causes a rapid decrease in the strength of the material.

What I claim as new and desire to secure by Letters Patent is:

1. A plain carbon steel of high elastic limit and sufiicient ductilityfor structural and similarpurposes as herein mentioned, in martensite and ferrite form and containing not over 0.25% carbon and between .5% and 1% manganese. 2. A plain carbon steel of high elastic limit and sufficiently ductile for structural and similar purposes, having a relatively small carbon content existing in a solid solution of iron carbide in beta iron and containing some free ferrite.

3. A method of producing a plain carbon steel of high elastic limit and sufliciently ductile for structural and similar purposes, as herein mentioned, which consists of heating a normal steel containing not over 0.25% carbon and between .5% and 1% manganese to a temperature above; transformatlon point, quenching rapidly, and reheating to a temperature below transformation point.

4. A method of producing a plain carbon v perat-ure not above 1300 Fahrenheit, maintaining this temperature for a period of from ten minutes to one hour, or longer as may be required, and thereafter cooling, the same slowly or quickly.

5. A plain carbon steel of high elastic limit and sufficiently ductile for structural and other hereinbefore mentioned purposes, having a martensitie or paitly martensitic. structure and containing not more than 0.25% carbon and between .5% and 1% manganese.

6. The method of increasing the elastic limit and yet retaining ductility of low carbon rolled or cast steel, which consists in subjecting plain carbon steel containing not more than 25% carbon and between .57, and 1% manganese, to heating at a temperature above the transformation point and for a period of time suflicient to bring about complete migration of the carbon across the ferrite grain, quenching the steel rapidly. and thereafter reheating to a lower temperature.

WELTON J. CROOK. 

